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040 _cIIA Library
080 _a043:52
100 _aAnupama, G. C.
245 0 _aStudies of classical and recurrent novae
_cG. C. Anupama
_h[Ph.D Thesis]
260 _aBangalore
_bIndian Institute of Astrophysics
300 _ax,173p.
502 _bDoctor of Philosophy
_cIndian Institute of Astrophysics, Bangalore
520 _aNovae belong to the cataclysmic variable class of objects, which includes dwarf novae, recurrent novae and classical novae. These systems undergo out bursts ranging from Δm ∼ 2-5 mag for dwarf novae, Δm ∼ 7-9 mag for recurrent novae, to Δm ∼ 9-> 14 mag for classical novae, with the inter-outburst periods being ∼ weeks to years for dwarf novae, ∼ decades for recurrent novae and ∼ 104 years for classical novae. Cataclysmic variables are interacting binary star systems consisting of a Roche-lobe filling secondary, on or near main sequence, losing hydrogen-rich material through the inner Lagrangian point onto an accretion disc that surrounds the primary, which is a white dwarf in most cases. A classical nova outburst is caused by a thermonuclear runaway on the surface of the white dwarf primary, whereas in dwarf novae the outburst is due to accretion disc instabilities, caused by factors such as enhanced mass transfer. Novae serve as valuable astrophysical laboratories. The physics of accretion onto compact, evolved objects, thermonuclear runaways on semi-degenerate surfaces, line formation and transfer processes in moving atmospheres, and formation of dust in the ejected matter are some of the astrophysical problems that can be understood better by detailed studies of novae. Such problems are also encountered in other instances of ejection of matter such as supernovae and planetary nebulae. In this study we present optical spectroscopic data of the classical novae LW Serpentis 1978, Nova Scuti 1989 and of the recurrent nova RS Ophiuchi 1985, obtained during outbursts. Spectroscopic data of the recurrent novae T Coronae Borealis, RS Ophiuchi and T Pyxidis and the classical nova GK Persei 1901 obtained during quiescence are also presented. Also, CCD images of the shells of GK Persei, and T Pyxidis are presented. Most of the data used in the study were obtained with the 102 cm reflector at the Vainu Bappu Observatory (VBO). In addition, some data obtained at the European Southern Observatory (ESO), kindly made available by H.W. Duerbeck and also some archival data from the International Ultraviolet Explorer (IUE), kindly made available by A. Cassatella are made use of. All data were reduced at VBO using the locally developed RESPECT software package, as also the STARLINK package with locally developed application routines. These data have been used to study the physical conditions in the nova envelopes, and the components of the binary system. Chapter I contains a general introduction to the field, and the outline of the dissertation. Chapter II describes the procedure of data reduction. Results on three classical novae are presented in Chapter III. The outburst spectrum of the moderately slow nova LW Serpent is 1978 compares well with that of a typical nova. Based on moderate-resolution Ha-line profile, a kinematical model for the shell of LW Serpentis is proposed. The spectrum of Nova Scuti 1989, also a moderately slow nova, compares well with LW Serpent is at similar epochs. The fluxes in emission lines have been used in determining the physical conditions in the ejected material. Spectra observed during oscillations in the light curve show that the variations are mostly in the continuum, and hence imply a change in photospheric radius. CCD images of the shell of nova GK Persei obtained in the lines of [N II] and [0 III] are used to determine the expansion of the shell by comparison with data available in literature. The shell is asymmetric with bulk of the emission arising in the southwest quadrant in the [N II] image. There is a difference in the distribution of [0 III] and [N II] emission indicating chemical. inhomogeneities. The spectrum at quiescence is decomposed into those of KO-2 IV secondary and the hot accretion disc. The mass transfer rate is estimated to be ∼ 10-10 С yr-1 . The He/H abundance in the accretion disc is ≤ 0.24. Results on three recurrent novae are presented in Chapter IV. Although the outburst characteristics of recurrent novae are similar to those of classical novae, the cause of the outburst in these systems is rather uncertain. Outburst in most cases appears to be due to thermonuclear runaways on massive white dwarfs. However, in the case of recurrent novae RS Ophiuchi and T Coronae Borealis, alternative explanations also exist, according to which the primary in these systems is a main sequence star, with outbursts attributed to instabilities in the disc. The fluxes in the Ha, He I and He II emission lines during the out burst of RS Ophiuchi have been used to determine the electron density and helium abundance (He/H= 0.16) in the envelope. Based on an estimate of the number of hydrogen and helium ionizing photons, the temperature and radius of the ionizing source have been determined. The results obtained during the late stages of outburst are consistent with the primary being a white dwarf rather than a main sequence star. The coronal line :B.uxes have been used to determine the temperature in the shocked ejecta. Spectra obtained during quiescence indicate that the secondary is an Mʘ ± 1 giant. The presence of strong 0 18446 Å emission line implies presence of Ly(3 fluorescence and a high temperature for the ionizing source. The presence of 0 I 7774 Å. line in absorption indicates that the accretion disc is optically thick. The quiescence spectrum is decomposed into the spectra of cool secondary and hot accretion disc. The mass transfer rate is estimated to be '" 10-6 Mʘ yr-1 . Spectra of the recurrent nova T Coronae Borealis obtained during its quiescence phase (1985-1990) show that the secondary is an M4 ± 1 giant. The emission lines in the spectra are variable in strength. The Ha flux shows a long term variation with a period ∼ 2400 days. Superposed over this is an orbital phase dependent variation with maxima at '" 0.5 and rv 0.9 phase. The estimated mass transfer rate is '" 10-7 Mʘ yr-1 . Images of T Pyxidis in [N II] and [0 III] reveal a bright shell ejected in the 1944 outburst and a faint extension due to the 1920 outburst. Comparison of VBO and ESO images shows that the bright shell expanded by 0.2 ± 0.1 arcsec in 3 years. The differences in [0 III] and [N II] images suggest the presence of chemical inhomogeneities as in the case of GK Persei. The spectrum at quiescence is dominated by the accretion disc, and has a high degree of excitation. The estimated mass transfer rate is ('V 10-8 Mʘ yr-1 , and HelH abundance is ≤0.24. The results of the study are summarized in Chapter V. A mention is also made of the problems that require further investigation using medium and large telescopes, and multiwavelength long~term monitoring. The tables, figures and equations are numbered sequentially in each section, with the chapter and section numbers indicated by suffixes.
650 _aAstronomy and Astrophysics Thesaurus
650 _aNovae
650 _aPh.D. Thesis
650 _aRecurrent Novae
700 _aT. P. Prabhu
856 _uhttp://prints.iiap.res.in/handle/2248/137
_yClick Here to Access eThesis
942 _cTD
999 _c10079